Current status of fertilization, distribution of N and P in soil profiles and techniques for reducing fertilizer application and improving efficiency in China’s apple orchards

Objectives The current status of fertilization and the ensuing N and P contents in soil profiles of China's main apple production areas were studied by reviewing published data, field surveys, and relevant laboratory analysis. We evaluated the techniques based on their potential to reduce fertilizer use, improve fertilizer use efficiency, and enhance soil fertility of apple orchards in China.

Methods The reasonable application rates of fertilizers were established according to the literature data; the status of chemical fertilizers applied in main apple-producing regions was determined by field surveys and literature data; the contents of NO₃^–-N and Olsen-P in soil profiles treated with and without irrigation were examined through field sampling and laboratory analysis; the techniques for reducing the application rates of fertilizers and increasing their efficiency, improving soil fertility, and underpinning high yield and quality for apple were integrated based on the literature.

Results According to literature, the ational chemical fertilization rates for apple orchards in China were N 150–420 kg/hm², P₂O₅ 90–330 kg/hm², and K₂O 120–420 kg/hm². However, the actual average rates were N 905 kg/hm², P₂O₅ 570 kg/hm² and K₂O 675 kg/hm², showing a severe over-application of N, P and K fertilizers. Moreover, farmers eferred synthetic fertilizers to organic fertilizer, with an organic nutrient proportion of only 7.0%. In the rainfed orchards, the NO₃^–-N content in 0–600 cm soil profile of the 8-year old apple orchards was similar to that in nearby farmlands, while the NO₃^–-N content in 20–500 cm soil depth in 25-year old orchards were significantly higher than that in farmlands, with a peak value of 215 mg/kg in the 120 cm soil layer. In irrigated 25-year old orchards, the NO₃^–-N content was higher than 100 mg/kg across the 800 cm soil profile, and the peak value was 265 mg/kg observed in the 380 cm soil layer. Notably, the NO₃^–-N contents in 140–600 cm soil depth in irrigated 25-year orchards were significantly higher than the value recorded for rainfed 25-year apple orchards. The soil Olsen-P contents decreased in 0–100 cm layer but increased in 100–400 cm, and stabilized in 400–600 cm. In rainfed orchards, the Olsen-P contents in 0–60 cm layer followed the trend of 25-year orchards>8-year orchards and farmlands, while in 60–600 cm soil depth, the differenc in Olsen-P contents was not significant among the land-use types. Interestingly, the Olsen-P content in the 60–120 cm soil depth of irrigated 25-year orchard soils was higher than the value recorded for the rainfed 25-year orchards, the peak value of 14.5 mg/kg was observed in the 80–100 cm soil layer, and similar changes of Olsen-P content were observed in 460–560 cm depth. In general, our findings showed that over and imbalanced fertilization was poplula in China’s apple orchards. Long-term excessive fertilization resulted in the considerable accumulation of NO₃^–-N and Olsen-P in deep soil profiles, and irrigation exaggerated their leaching, resulting in their unavailability for plant growth.

Conclusions Fertilzation and nutrient expert system for apple can effectively reduce both water and nutrient input. Increasing organic fertilizer proportion can increase the even supply of nutrients. Grass covering and biochar application efficiently hold nutrients, improve organic matter content and fertile soil. Future studies should focus on the integrated application of the techniques that are suitable for different ecological regions.